Microwave oven



n-3,1961 B. R. MCAVOY Em 3,296 5 MICROWAVE OVEN Filed Dec. 23. 1963 2Sheets-Sheet 1 MICROWAVE GENERATOR WAVEGUIDE OVEN Fig. l

INVENTORS Bruce R. Mc Avoy and BY Thaddeus A. Osiol. RM? KM ATTORNEYJan. 3, 1967 R. McAVOY ETAL 3,296,405

MICROWAVE OVEN Filed Dec. 23. 1965 2 Sheets-Sheet 2 Fig.5.

Fig.8.

United States Patent 3,296,405 MICROWAVE OVEN Bruce R. McAvoy,Pittsburgh, and Thaddeus A. Osial,

Penn Hills, Pa., assignors to Westinghouse Electric Corporation,Pittsburgh, Pa, a corporation of Pennsylvania Filed Dec. 23, 1963, Ser.No. 332,748 6 Claims. (Cl. 219-1055) This invention relates to heatingof materials by ex posure to electromagnetic radiation and concernsespecially distribution of microwave energy for uniform heating therebyin an oven or similar enclosure.

Increase in temperature of materials exposed to electro magneticradiation in the microwave portion of the spectrum (from about onethousand to three hundred thousand megacycles per second, correspondingto wavelengths of from about thirty centimeters to one millimeter) is afamiliar, phenomenon. Microwave generators operating in the lowmidportion of this range are used to provide energy to cooking ovens orthe like. However, the pattern of radiation established in an oven, forexample,

. resembles standing waves to such an extent that the food or othermaterial to be heated is affected non-uniformly. For this reason it iscustomary to stir the pattern or mix the modes by means of a rota-tingfan having electrically conductive blades to reflect the radiationintermittently. However, the added initial cost and upkeep of such adevice are unwelcome, and there is a great need for improvement.

A primary object of the present invention is assurance of heatinguniformity in a microwave oven or the like without use of moving bladestherein.

Another object is coupling of microwave energy from a waveguide into anoven or similar enclosure by stationary means effective to smooth oreven out the energy pattern therein.

A further object is accomplishment of the previous objects withoutsignificantly decreasing the useful volume of the oven or likeenclosure.

Other objects of this invention, together with means and methods forattaining the various objects, will be apparent from the followingdescription and the accompanying diagrams.

FIG. 1 is a block diagram of microwave heating apparatus;

FIG. 2 is a perspective view, partly diagrammatic, of components of thepreceding view constructed according to one embodiment of the presentinvention;

FIG. 3 is a view similar to FIG. 2 but with the components constructedaccording to another embodiment of this invention;

FIG. 4 is a diagrammatic representation of electrical elementsencountered in the practice of the invention;

FIG. 5 is a perspective view, on an enlarged scale, of a portionencircled and designated V in FIG. 2;

FIG. 6 is a front sectional elevation of a portion of the apparatus ofFIG. 3, with a diagrammatic representation of electrical wiring added,taken as indicated at VIVI on FIG. 7;

FIG. 7 is a side sectional elevation of the portion of ap aratus shownin FIG. 6, taken at VII-VII thereon; and

FIG. 8 is a perspective view of a part of the portion of apparatus shownin FIGS. 6 and 7.

In general, the objects of the present invention are accomplished, inapparatus for heating materials by exposure to microwave energy in anenclosure supplied therewith from a source outside the enclosure, bymeans for conducting the energy from the source and coupling it into theenclosure at a plurality of locations therealong, microwave switchingmeans for varying the coupling impedance at the respective locations,and means for actuating the switching means variously from time to timeand thereby varying the energy input to the enclosure at the respectivelocations so as to modify the energy pattern in the enclosure. Theinvention contemplates use of semiconductor (ferro electric) diodes as,or in, the microwave switching means.

FIG. 1 shows, in the form of a block diagram, the interconnection of amicrowave generator to an oven or the like by means of a waveguide.Specific constructions useful according to the present invention areshown in the subsequent views.

FIG. 2 shows, in perspective, an oven 11 viewed from below and awaveguide 12 viewed from above, spaced from one another along phantomlines indicating the manner of assembling the one to the other. Thebottom wall or floor of the oven has a pair of circular openings orports 13 therein spaced similarly to and opposite a pair of likeopenings 14 in a terminal portion 15 of the waveguide, which is joinedto the major portion 16 thereof. Shown schematically across openings 14are a pair of diodes 18 (one across each of the openings). An encircledportion marked V, including one such diode, is shown considerablyenlarged in FIG. 5, which also shows a collar 17 surrounding anddefining the opening 14 across which the diode 18, at the end of anelectrical lead 19, extends to terminate against the inside wall of thecollar. This completes the illustration of a first structuralembodiment, as compared with a second embodiment illustrated in FIGS. 3,6, 7, and 8.

FIG. 3 shows, in like manner as FIG. 2, an oven 21 viewed from below anda waveguide 22 viewed from above, spaced from one another along phantomassembly lines. Rectangular opening 23 in the bottom wall or floor 31 ofthe oven is the same size, shape, and orientation as opposingrectangular aperture 24 near the end of the waveguide. Intruding upwardinto the oven is a structure having a rectangular roof 26 supported on apair of trapezoidal end walls 25, '25. Defined thereby and by the floorof the oven alongside the opening 23 are a pair of oblique rectangularports 27, 27' crossed by a pair of diodes 28, 28', shown schematically.Further details of this embodiment appear in FIGS. 6, 7, and 8.

FIG. 6 shows in front sectional elevation, and FIG. 7 in side sectionalelevation, a bot-tom portion of the oven 21 and terminal portion of thewaveguide 22 assembled to one another, together with a diagrammaticshowing of associated electrical wiring (in FIG. 6). FIG. 8 shows, inperspective, the structure composed of roof 26 and supporting end walls25, 25 intruding upward from the floor of the oven, shown in frontelevation in FIG. 6 and'side sectional elevation in FIG. 7. Theintruding structure is made of electrically conductive material as arethe bottom or floor 31, walls, and top or root (not shown) of the oven.

The oven 21 has a false bottom or floor 32 of dielectric material, whichis essentially transparent to microwave radiation, extending from sideto side a short distance above the roof 26 of the intruding structure.The diodes 28, 28 extend obliquely upward from their leads 29, 29' tocross the ports 27, 27', which they bisect laterally, and terminate intapered ends abutting the opposite edges of the roof 26. The leads 29and 29' terminate at respective terminals a and b, across which avarying electrical potential (indicated piotorially by a sine wave) isimpressed; lead 39 from the bottom or floor 31 of the oven is grounded,as indicated, at terminal 0 located midway (electrically) between theother two. Although not so shown. like wiring is suitable for thepreviously described and illustrated embodiment. In either event theleads to the diodes may be brought out of. the oven cavity through acapacitive bypass to minimize leakage of microwave energy from the oveninterior.

Although not indicated in FIG. 6, each of the diode leads 29, 2? willhave, as indicated in the schematic diagram in FIG. 4, a lead resistanceR a lead inductance L which must be considered together with a. junctionresistance R and a junction capacitance C attributatahle to theconnection of the diode to the oven. R L and C though variable as amatter of design, are substantially constant in any single design, whileR is adapted to vary between two markedly different values dependingupon the diode biasing.

When either diode is biased in the forward direction RJ is small,effectively shunting C and the diode (with lead) impedance is accountedfor by R and L When the bias is in the reverse direction, however, R, isquite large so that Q, must be considered. The inductive reactance.

of the lead may make only a negligible contribution to the totalimpedance, or it may be adjusted to provide series resonance with thejunction capacitance.

When a diode is forward-biased, microwave energy passes through theinput port bridge thereby without significant attenuation; however, whenthe diode is reversebiased, relatively little microwave energy passesthrough the port and into the oven. Accordingly, alternating the bias ofthe respective diodes at a frequency low with respect to that of themicrowave energy (e.g., ordinary power frequencies of 50 or 60 c.p.s.)elfectively switches the input energy from one input port to another. Asdesired, the pattern of energy in the oven is mixed or stirred therebyto eliminate standing waves, evening out the heating effect upon exposedfoodstuffs or the like.

The microwave switching means utilized according to this invention may(and presumably will) be conventional. Junction (ferro-electric) diodeshaving a power rating sufficiently high to handle the applied load aresatisfactory. They may be made of any appropriate semiconductingmaterial adapted to such use, and their selection and incorporation intothe apparatus of this invention will be well within the ability ofpersons having ordinary skill in the art.

The apparatus of this invention may .be modified by alteration in size,shape, number, and state of subdivision or combination of parts orotherwise while retaining benefits and advantages of the invention asdefined in the claims.

We claim as our invention:

1. In a microwave oven or the like having an external source ofmicrowave energy, a channel-like indentation open to the exterior andhaving oblique opposite side walls with a separate opening in each ofsaid opposite sidewalls thereof adapted to admit microwave energy to theoven interior, separate electronic microwave switching means at eachopening adapted to vary the impedance presented to incident microwaveenergy, means for conducting microwave energy from the external sourceto the channel-like indentation, and means for controlling saidswitching means to alternately substantially pass, and alternatelysubstantially attenuate, the microwave energy at each said separateopening.

2. In a microwave oven or the like having an external source ofmicrowave energy, a channel-like indentation open to the exterior, withoblique sidewalls, and with a separate opening in each of a pair of theoblique sidewalls thereof adapted to admit microwave energy to the oveninterior, wave-guide means for conducting microwave energy from theexternal source to the channel-like indentation, separate electronicmicrowave switching means including semiconductor diodes at therespective openings adapted to vary the impedance presented to incidentmicrowave energy, and means for biasing the respective diodesalternately forward and backward at a relatively low frequency toalternately substantially pass to said oven interior, and alternatelysubstantially attenuate, the microwave energy at each said opening.

3. In apparatus for heating materials by exposure to microwave energy inan enclosure supplied therewith from a source outside the enclosure,

single passage means for conducting the energy from the source andcoupling it into the enclosure at a plurality of separate but closelyadjacent inlet locations to said enclosure, said enclosure being ofsubstantially greater dimensional extent that said passage means,

separate electronic microwave switching means at each said inletlocation for varying the coupling impedance at each said location,

means for actuating each said separate switching means at its respectivelocation to substantially block and alternately substantially passmicrowave energy to said enclosure,

said separate switching means being actuated alternately with respect toeach other to switch the input of said energy between said inletlocations to modify correspondingly the energy pattern in saidenclosure.

4. An apparatus according to claim 3,

said conducting means is in the form of a rectangular waveguide.

5. An apparatus according to claim 3,

each said microwave switching means comprises a semiconductor diode, and

said means for actuating said switching means includes means for biasingeach said diode alternately forward and 'backwardly,

6. An apparatus according to claim 5,

sad biasing means includes a source of relatively lowfrequencyalternating potential, series load resistance and a ground connectionfor the enclosure to which one side of said diodes is connected.

References Cited by the Examiner UNlTED STATES PATENTS 2,790,054 4/1957Haagensen 21910.55 3,014,188 12/1961 Chester et al. 33383 3,017,5851/1962 Luke 333-7 3,127,494 3/1964 Kellough et al. 219-10255 3,178,6594/1965 Smith et al. 333-7 RICHARD M. WOOD, Primary Examiner.

ANTHONY BARTIS, Examiner.

L. HJBENDER, Assistant Examiner.

1. IN A MICROWAVE OVEN OR THE LIKE HAVING AN EXTERNAL SOURCE OFMICROWAVE ENERGY, A CHANNEL-LIKE INDENTATION OPEN TO THE EXTERIOR ANDHAVING OBLIQUE OPPOSITE SIDE WALLS WITH A SEPARATE OPENING IN EACH OFSAID OPPOSITE SIDEWALLS THEREOF ADAPTED TO ADMIT MICROWAVE ENERGY TO THEOVEN INTERIOR, SEPARATE ELECTRONIC MICROWAVE SWITCHING MEANS AT EACHOPENING ADAPTED TO VARY THE IMPEDANCE